How to Calculate Six Sigma
What is Six Sigma?
Measures process consistency using statistical defect rates. Six Sigma means 99.99966% of outputs meet specifications.
Step-by-Step Guide
- 1Measure process output distribution (mean, standard deviation)
- 2Define upper and lower specification limits
- 3Calculate sigma level: (specification limit - mean) ÷ std deviation
- 4Fewer defects = higher sigma level
Worked Examples
Input
1M parts, 3.4 defects
Result
6-sigma
Common Mistakes to Avoid
- ✕Not accounting for process drift over time
- ✕Using non-normal distributions without transformation
Frequently Asked Questions
What sigma level is acceptable?
3-sigma: 66,000 defects per million; 4-sigma: 6,200 defects; 6-sigma: 3.4 defects per million.
Is Six Sigma achievable?
Few processes achieve true 6-sigma; most aim for 4-sigma; improvement is gradual.
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